The present invention relates to novel friction compositions containing aramid fibers, and more particularly to automotive clutch facings and like friction elements made from such compositions.
The automotive industry is ever in search of improved friction materials for use in automobiles, trucks, and other motor vehicles. One particular area where improved friction materials have been sought is in the design and fabrication of clutch facings. Criteria of particular importance in clutch facing design are a burst strength adequate to meet ever higher rotational speeds of vehicle drive chains, an adequate coefficient of friction, and reduction in wear to increase the useful life of clutches.
Major manufacturers of automotive clutch facings require that clutch facings meet a particular performance standard in the hot burst strength test. In this test, the driven member of the clutch, with two facings attached, is heated to 500.degree. F. for 15 minutes and then be subjected to spin testing as a prerequisite to product acceptance. This test, is one of the most difficult tests to be passed by a clutch facing since at elevated temperatures fibrous materials and the binder comprising resin and/or rubber of which the clutch facing is formed lose strength, resulting in bursts at relatively low rotational speeds. Conventional clutch facings formed of asbestos fibers having an OD of 11 inches and an ID of 61/2 inches are known to withstand spin speeds of about 8,000 to 9,000 r.p.m. in the hot burst strength test.
As spin requirements for clutch facings have become more demanding, glass fibers have been used to strengthen the facings while asbestos has been retained as the friction material.
More recently, in an effort to improve hot burst strength, clutch facings have been formed from bundles of parallel continuous glass filaments spirally or randomly wound upon themselves in the form of an annular disc and impregnated with heat-curable cement. Such constructions are described in U.S. Pat. Nos. 3,743,069 and 3,756,910.
According to U.S. Pat. No. 4,130,537, glass, as the active friction ingredient of a friction element, is too "aggressive" for most commercial applications, the aggressiveness being manifested during use by noise, vibration and/or erratic friction effects when the friction element is engaged with a mating surface. In order to overcome this shortcoming, the patent teaches the inclusion of infusible organic fibers having a decomposition temperature above 400.degree. F. but below 800.degree. F. Aramid fibers sold by duPont under the name Nomex are given as an example of such fibers.
Conventional clutch facings formed of asbestos fibers, although having adequate friction properties, e.g. a coefficient of friction of from about 0.22 to about 0.44, in addition to having limited hot burst strength, also exhibit a relatively high rate of wear on the order of about 0.01-0.012 inches for each 10,000 engagements. Although glass fiber clutch facings exhibit improved hot burst strength and somewhat reduced wear, as compared to asbestos fiber clutch facings; nevertheless, there continues to be a need for improvements in both of these essential properties.
It is an object of this invention to provide a novel friction composition containing aramid fibers which is particularly suitable for preparation of automotive clutch facings and brake linings.
Another object of the invention is to provide clutch facings composed of aramid fibers and possessing increased hot burst strength and improved wear properties.